Put all firmware in $out/lib/firmware

This way, hardware.firmware can be a list of packages.

nixos/modules/hardware/all-firmware.nix

···
   ###### implementation
 
   config = mkIf config.hardware.enableAllFirmware {
-    hardware.firmware = [
-      "${pkgs.firmwareLinuxNonfree}/lib/firmware"
-    ];
+    hardware.firmware = [ pkgs.firmwareLinuxNonfree ];
   };
 
 }

nixos/modules/hardware/video/encoder/wis-go7007.nix

···
 in
 
 {
-  boot.extraModulePackages = [wis_go7007];
+  boot.extraModulePackages = [ wis_go7007 ];
 
-  environment.systemPackages = [wis_go7007];
+  environment.systemPackages = [ wis_go7007 ];
 
-  hardware.firmware = ["${wis_go7007}/firmware"];
+  hardware.firmware = [ wis_go7007 ];
 
-  services.udev.packages = [wis_go7007];
+  services.udev.packages = [ wis_go7007 ];
 }

nixos/modules/services/hardware/udev.nix

···
     };
 
     hardware.firmware = mkOption {
-      type = types.listOf types.path;
+      type = types.listOf types.package;
       default = [];
       description = ''
-        List of directories containing firmware files.  Such files
+        List of packages containing firmware files.  Such files
         will be loaded automatically if the kernel asks for them
         (i.e., when it has detected specific hardware that requires
-        firmware to function).  If more than one path contains a
-        firmware file with the same name, the first path in the list
-        takes precedence.  Note that you must rebuild your system if
-        you add files to any of these directories.  For quick testing,
+        firmware to function).  If multiple packages contain firmware
+        files with the same name, the first package in the list takes
+        precedence.  Note that you must rebuild your system if you add
+        files to any of these directories.  For quick testing,
         put firmware files in <filename>/root/test-firmware</filename>
-        and add that directory to the list.  Note that you can also
-        add firmware packages to this list as these are directories in
-        the nix store.
+        and add that directory to the list.
       '';
       apply = list: pkgs.buildEnv {
         name = "firmware";
         paths = list;
-        pathsToLink = [ "/" ];
+        pathsToLink = [ "/lib/firmware" ];
         ignoreCollisions = true;
       };
     };
···
       (isYes "NET")
     ];
 
-    boot.extraModprobeConfig = "options firmware_class path=${config.hardware.firmware}";
+    boot.extraModprobeConfig = "options firmware_class path=${config.hardware.firmware}/lib/firmware";
 
     system.activationScripts.udevd =
       ''
···
 
         # Allow the kernel to find our firmware.
         if [ -e /sys/module/firmware_class/parameters/path ]; then
-          echo -n "${config.hardware.firmware}" > /sys/module/firmware_class/parameters/path
+          echo -n "${config.hardware.firmware}/lib/firmware" > /sys/module/firmware_class/parameters/path
         fi
       '';
 

nixos/modules/system/activation/top-level.nix

···
 
         ln -s ${config.system.build.initialRamdisk}/initrd $out/initrd
 
-        ln -s ${config.hardware.firmware} $out/firmware
+        ln -s ${config.hardware.firmware}/lib/firmware $out/firmware
       ''}
 
       echo "$activationScript" > $out/activate

nixos/modules/system/boot/kernel.nix

···
       ];
 
     # The Linux kernel >= 2.6.27 provides firmware.
-    hardware.firmware = [ "${kernel}/lib/firmware" ];
+    hardware.firmware = [ kernel ];
 
     # Create /etc/modules-load.d/nixos.conf, which is read by
     # systemd-modules-load.service to load required kernel modules.

pkgs/os-specific/linux/firmware/b43-firmware/5.1.138.nix

···
   phases = [ "unpackPhase" "installPhase" ];
 
   installPhase = ''
-    mkdir $out
-    b43-fwcutter -w $out linux/wl_apsta.o
+    mkdir -p $out/lib/firmware
+    b43-fwcutter -w $out/lib/firmware linux/wl_apsta.o
   '';
 
   meta = {

pkgs/os-specific/linux/firmware/zd1211/default.nix

···
   version = "1.5";
 
   name = "${pname}-${version}";
-  
+
   src = fetchurl {
     url = "mirror://sourceforge/zd1211/${name}.tar.bz2";
     sha256 = "04ibs0qw8bh6h6zmm5iz6lddgknwhsjq8ib3gyck6a7psw83h7gi";
   };
-  
+
   buildPhase = "true";
 
-  installPhase = "mkdir -p $out/zd1211; cp * $out/zd1211";
-  
+  installPhase = "mkdir -p $out/lib/firmware/zd1211; cp * $out/lib/firmware/zd1211";
+
   meta = {
     description = "Firmware for the ZyDAS ZD1211(b) 802.11a/b/g USB WLAN chip";
     homepage = http://sourceforge.net/projects/zd1211/;

pkgs/os-specific/linux/wis-go7007/default.nix

···
     mkdir -p $out/etc/udev/rules.d
  
     makeFlagsArray=(KERNELSRC=${kernel.dev}/lib/modules/${kernel.modDirVersion}/source \
-        FIRMWARE_DIR=$out/firmware FXLOAD=${fxload}/sbin/fxload \
+        FIRMWARE_DIR=$out/lib/firmware FXLOAD=${fxload}/sbin/fxload \
         DESTDIR=$out SKIP_DEPMOD=1 \
         USE_UDEV=y)
   ''; # */